Tone control circuit



May 2, 1950 c. D. KNIGHT TONE CONTROL CIRCUIT Filed Aug. 27, 1945 IN VEN TOR.

w 5 N. W m M T w 5 9 m8 Patented May 2, 1950 TONE CONTROL CIRCUIT Cosler Donald Knight, Chicago, Ill., assignor to Zenith Radio Corporation, a corporation of Illinois Application August 27, 1945, Serial N 0. 612,926 1 Claim. (01. 179-171) The present invention relates to amplifying circuits and more particularly to novel circuit arrangements for improving tone quality of signals applied to such circuits, for example, in radio transmitters, receivers, public address systems, phonographs, etc.

.In the so-called table model radio receiver having a relatively small speaker and small cabinet, low frequency audible notes are more difficult to faithfully reproduce than are the high frequency audible notes and as a consequence one may in most present day receivers ascertain whether it is a so-called table model or floor model by the low frequency response thereof. Inasmuch as the size of a radio receiver including its speaker and cabinet influences greatly its ability to reproduce low frequency audible notes, it is desirable that there be some compensation provided for the deficiency in reproducing the low notes.

It is therefore an object of the present invention to provide a circuit for use in relatively small radio receivers to make them sound like large floor model receivers of the type which have a relatively large speaker and a relatively large cabinet which acts at least in part as a sounding board for low frequency audible notes.

Furthermore, it is desirable to produce compensation for loss of low frequency audible notes or bass boost by utilizing a regenerative circuit instead of a degenerative circuit since in the former case greater over-all gain may be realized for the same number of parts than is realized in the latter case wherein some of the available over-all gain is inherently suppressed. While the present invention relates particularly to arrangements for providing bass boost or'compensation of loss in small table model receivers, the present invention finds applicability also in large floor model receivers where it is desired, for example, to reproduce predominantly the low frequency audible notes.

Another object of the present invention is to provide an improved bass boost circuit arrangement for universal use in any electrical system having currents therein of audible frequency.

Still another object of the present invention is to provide an improved and inexpensive bass boost circuit utilizing a minimum number of parts for use in radio receivers made in accordance with present day mass production methods.

Yet another object of the present invention is to provide an improved arrangement of a regenerative "bass boost circuit which increases the over-all gain of the associated apparatus.

1 The features of the present invention which wave is impressed on a volume control resistance connected in the input circuit of an amplifying device, at least one element of a low frequency responsive circuit being connected with the volume control resistance and serving also as a common coupling element in a feedback circuit regeneratively connected to the output circuit of the amplifying device.

Referring more specifically to Fig. l, signals applied, for example, to the secondary winding- I of an intermediate frequency transformer of a radio receiver, are demodulated in rectifying device 2, amplified in amplifying device 3, further amplified in amplifying device 4, and then ap-. plied to speaker 5, a relatively small portion of the signal amplified by device 4 being fed back regeneratively from the output circuit of device 4- to the input circuit of device 3 through a path including condenser 6, resistors 1 and 8, volume control resistance 9 and condenser l0.

Secondary winding I, tuned by condenser I I to the carrier wave frequency on which audio signals are modulated, is serially connected in conven-;

tional fashion to the rectifying device 2 through a load resistance l2, the junction point'of one terminal of resistance l2 and the cathode of device 2 being grounded as indicated at 13. The other terminal of resistance I2 is connected to the upper or ungrounded terminal M of volume control resistance 9 through a coupling condenser [5 for, the passage of audio frequency currents, currents of carrier wave frequency being filtered by filter condenser l6 connected in shunt to resistance l2.

Volume control resistance 9 has its lower terminal l1 grounded, its variable tap 18 connected to the control grid of device 3 through condenser l0 and a fixed point I9 thereon connected to one terminal of resistance 8, the other terminal of resistance 8 being connected to ground through" condenser 20 which serves important dual func v tions as explained below.

Grid leak resistance 2i connected between the main control electrode of device 3 and the grounded cathode of device 3 serves to provide a. proper bias voltage in accordance with wellknown principles.

The anode of device 3 is connected to control grid 22 of power amplifier device 4 through coupling condenser '23 which serves also to block flow of space'current which would otherwise flow to grid 22 from the positive terminal of the anode voltage supply 22A through the anode resistance 24 serially connected between such terminal and the anode of device 3. The anode voltage source has its negative terminal grounded, :for example, at point 25. 1

Grid leak resistance 26 is connected between control electrode 22 and ground, control electrode 22 being maintained at a suitable negative potential by space current flowing through resistance 21 which is connected between ground and cathode 28and bypassedfor audio-frequencies by condenser 129.

iiifirst portionof the signal developed on anode 30,,is transferred to the primary winding 3| of output transformer. 32 connected between anode 3.0 and the positive terminalof source 22A, while a secondrelatively small portion .ofsuch signal is transferred back to the input circuit of device 3 in regenerative phase through the network 6, l which is serially connected betweenanode 3i and thejunction pointof resistance 8 and condenser 20. Such first signal portion from anode fat is appliedto loudspeaker connected to thesecondary winding of transformer 32,whi1e such secondsignalportion from anode '30 servesto increase the over-all gain of the apparatus and particularly to increase the over-all gain for-signals of low frequency to provide a compensation for the failure of speaker 5 and its. associated cabinet; to reproduce low frequency notes as well as high frequency notes.

Amplifying device 4 is preferably of they screen grid'type having its screen electrode 33 connected to the positive terminal of voltage source 22A for thefiow .of continuous current and connected to cathode .28 through condenser 3 to prevent the appearance. of signal voltage on screen electrode 3.3.

The circuit comprising volume control potentiometer .type resistance 9, resistance 8. and condenser -2 0-is termed a bass boost circuit conheated with the control electrode of device 3. That is, inasmuch as condenser 26 connected in parallel witha portion of volume control resistance=9has a reactance relatively'high for signals of lowaudio frequency and a reactance relatively low-for signals of high audio frequency, a proportionately greater voltage is developed at the control electrode-of device 3 for low frequency-signals than-for high frequency signals of the same intensity appearing on resistance l2. Thus condenser ;20 serves one important ,function apart from-the regenerative circuit including resistance 1 and condenser ii of which it also forms a part.

The magnitude of condenser 5 in relationship to the other, circuit parameters, especially resistance I and condenser 2%, should be large enough to-allow the transmission of signals of large intensity in the desired audible frequency range above, for example, 90 cycles per second, and. shouldbe small enough in magnitude to limit considerably signals of, for example, 60 cycles and less. If condenser 6. were not of suitable .small magnitude, the nonuseful audio frequencies crease the response for low audio signals over the corresponding response for high audio signals.

Certain ones of the circuit elements shown herein may, forexample, have the following magnitudes: Volume control potentiometer type resistance 500,000 ohms, the resistance between points I! and I9 thereon being 100,000 ohms; re-

' sistances l, B and 2! being respectively 8.2 megohms, 22,000 ohms and 15 megohms and condensers 6, ill and 2tbeing respectively ..0 3,.. .0.04 and .il-microfarads.

It-is noted that the amount of bass boost the circuit described varies withthe position-o1: the tap xisfrom point Mto point l1. This variation is portrayed graphically in Fig.2 wherein relative positions of the .tap it are-plotted linearly as abscissae and the corresponding magnitude of the regeneration ratio in the circuit is; plotted linearly asordinates, theregeneration ratiobeing defined as the ratio between the outputvsignal at anode 3% obtained with no regeneration-andthe' corresponding output signal at anode 30 obtained with regeneration.

It is desirable that the pointB on theregeneration ratio curve corresponding to point 19- on thepotentiometer type -resistance.9 has a small ordinate by making point ill as close to :-point II- as possible without altering seriously the :-bass boost response obtained from use of the network 9, 8,-29perse. I'he'smaller the ordinateof point B, the smaller-becomes its abscissa measured from-point C and the-less noticeable to an ob--- server when he adjusts the volumecontrolpotenticrneter type resistance 9 becomes the frequencyresponse change. It-isof course desirable that a change in volume control setting should not produce abrupt changesin the frequency response characteristics.

Whileparticular embodiments of the present invention have been .shown and described, it: will be obvious'to those skilled in the art that changes and modifications may be made without departing from this invention in its broader. aspects, and, therefore, the aiminthe appended claim-is to cover all such changes and modifications: as fall within the true spirit and scopeotthis :in v'ention.

I claim:

A-n audio-frequency amplifying system comprising: an electron-discharge device having a cathode, an input-electrode, andanoutputelectrode; e, volume control potentiometer having a variable taper-id a second tap; means coupling one terminal of said potentiometer to saidtcathode; means coupling said variable tap to said input electrode; an output circuit coupledtosaid output electrode and to said cathode;la frequency selective circuit comprising series-connected =im-. pedance elements coupled to said output-circuit and including a capacitance one terminal .:of which is coupledltosaid cathode; and means @including a resistance coupling theother terminal of said capacitance tosaid second: taplto'complete a feedback pathior supplying a portion-of the voltage developed across said capacitance in regenerative phase to said input electrode, whereby said capacitance functions simultaneously to provide low-audio-frequency emphasis and highaudio-frequency deemphasis.

COSLER DONALD KNIGHT.

REFERENCES CITED The following references are of record in the file of this patent:

Number UNITED STATES PATENTS Name Date Masters Oct. 8, 1940 Bruck et a1 July 14, 1942 Shepard, Jr Mar. 9, 1943 Bond Apr. 20, 1943 Crowley May 18, 1943 Fayers Jan. 9, 1945 Barton Apr. 17, 1945 

